A social network generally includes a network of individuals forming an association with one another based on his/her identity. Social networks may have different features according to differing characteristics and interest of their constituent members. In some respects, the social network may be regarded to a relatively high level due to the fact that humans living in social relationships may be affected by valuation and personal connections of the individual in his/her life. Also, social network are generally becoming more popular as a useful means for building new relationships for individuals.
However, conventional social network services often merely function to transmit and receive messages based on the established network between user terminals, and thus may not satisfy user's demand for various services.